. Animal physiology. Physiology, Comparative. DEVELOPMENT OF CIRCULATING APPARxVTUS. 589 Avails of both, the materials requisite for its growth ; hut there is no direct communication between the two. The same means serve for the aeration of the blood of the embryo; for this, being brought from its body in the venous condition, is exposed to the influence of the arterial blood of the parent, through the thin walls of its vessels,—^just as the venous blood of aquatic animals is aerated in their gill-tufts,—and passes back to the embryo in the arterial condition, having imparted its carbonic acid
. Animal physiology. Physiology, Comparative. DEVELOPMENT OF CIRCULATING APPARxVTUS. 589 Avails of both, the materials requisite for its growth ; hut there is no direct communication between the two. The same means serve for the aeration of the blood of the embryo; for this, being brought from its body in the venous condition, is exposed to the influence of the arterial blood of the parent, through the thin walls of its vessels,—^just as the venous blood of aquatic animals is aerated in their gill-tufts,—and passes back to the embryo in the arterial condition, having imparted its carbonic acid to the blood of the parent, and received from it oxygen.— Thus all but the very early stages of development are performed in Mammals, by means of which we scarcely find a trace in Oviparous animals ; yet the ova of both are originally formed on the same plan, and the first changes which they undergo are exactly analogous. 762. It would not be consistent either with the design or with the limits of this work, to enter in much detail into the considera- tion of the processes of develop- ment, although they present many points of the highest interest. The general history of the evolu- tion of the Circulating apparatus and of the .ISTervous centres may, however, be noticed, as character- istic examples of the mode in which the evolution of the several organs of the body takes place.— The Heart, in Man and other Mammals, as in the Bird (§ 758), is at first a simple tube, resembling the pulsatile trunk that remains as the sole organ of impulsion in the lowest forms of circulating apparatus. After a time this tube is doubled upon itself, and two cavities are formed, an auricle and a ventricle ; in this con- dition, it strongly resembles the heart of the Fish (§ 286). The circulation too is, at an early period, that of the Fish; for the. Fisf. 326.—Embryo of the Fowl, from the Ovum shown in fig. 318, greatly enlarged : a, b, folds of t'crminal membrane, envelojjing head an
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